Noise reducing circuits



Aug. 24, 1948.

C. W. CARNAHAN NOISE REDUCING CIRCUITS Filed Jan. '30, 1945 4 Sheets-Sheet 1 OED( ug- 24, 1948 c. w.. ;AR|\1AHAl\av NoIsE REDUCING CIRCUITS 4 Sheets-Sheet 2" Filed Jan. 50, 1943 INVENTOR.

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NOISE REDUCING CIRCUITS Filed Jan. so, 1945 4 sneeis-sheet 4 AUDIO FREQUENCY AMPLIFIER v IN VEN TOR. CHALON WESLEY CARNAHAN BY )L .M HIS AT ORNEY I fr) m Patented Aug. 24, 1948 NOISEi REDUCING CIRCUITS Chalon Wesley Carnahan, Evanston, Ill., assigner to Zenith Radio Corporation, a corporation of Illinois Application January 30, 1943, Serial No. 474,066v

(Cl. Z50-20) 29 Claims. l

'Ihis invention relates to signalingvapparatus and more particularly to a control network for the reduction of noise in high frequency signaling apparatus.

The performance of. radio receivers is commonly limited by the presence of noise. Static crash and tube hiss are well knownv examples of unpleasant noise, but any undesired disturbance, Whether originating within or without the receiver, is properly termed noise.

Noise limits the usable sensitivity of a radio receiver and necessitates a higher level of signal strength for satisfactory communication than is otherwise required. The intelligibility of signals may be reducedby severe background noise to such an extent that even commercial pointv to point Icommunication is interrupted. It is well known that in ordinary broadcasting, noise frequently disturbs the listenery and destroys the entertainment of programs. Noise interference must especially be minimized when operating mobileequipment, which operates underl conditions far less favorable than the worst faced by a broadcast receiver in the home.

It is a main object of this invention to provide a new and improved noise reducing arrangement.

Although the frequency modulation system of radio transmission makes possible the satisfactory reception of signals under extremely unfavorable noise conditions, it too is subject under certain conditions to undesirable noise interference. Unfortunately, a normal frequency modulation receiver in the absence of a signal carrier is operating at its maximum sensitivity. Under such a condition, even though the limiter stages are operating in a satisfactory manner and the demodulator is carefully balanced, the background noise is unpleasant and disturbing. When a broadcast receiver is tuned away from a carrier, a rush ofv hiss', crackle and sputter is received in disagreeable contrast toV the quiet background of a receiver correctly tuned to a signal of satisfactory level.

The noisy operating characteristics of a sensitive frequency modulation receiver in the absence of a carrier signal are particularly objectionable in the case of communication equipment with which the operatorcommonly wears headphones.

Quite often, a receiver must remain tuned for f long periods of time to astation which transmits a signal carrier only during the brief moment when messages are being sent. The crash and hiss of background noise deadenthe operators auditory perception and-createl a nervous strain which reduces his` operating efficiency.

Accordingly, it is an object of my invention to lprovide a new and simplified silencing networkv particularly adapted-for use witlra frequency modulation receiver which will prevent the reproduction of noise when no signal is being received, or when the signal intensity is too low for satisfactory use.

Certain silencing networks of the type commonly termed squelch circuits are known to the art. These noise reducing circuits are more properly designated by such terms as carrier off noise Suppressors or interchannel noise suppressors. Some of the prior art circuits have been adapted for use with frequency modulation receivers. In all cases observed, the operating characteristics necessary for satisfactory noise reducing action have been obtained only by recourse to relatively complicated and expensive circuit apparatus.

It is an object of thepresent invention,l therefore, to provide a new and improvedA noise reducing circuit which can be applied` to a conventional frequency modulation receiver. without the addition of extra tubes.

It is also an object of my invention to provide a simple and effective noise reducing circuit which llends itself to economical commercial manufacture.

It is a further object of my invention toprovide a new and improved network which will permit a radio receiver to become operative in the presence of a Very low level of frequency modulated signal, but which holds the receiver inoperative in the presence of a much higher level of noise alone inthe absence of receivable signal.

The features of myinvention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Figure 1 diagrammatically represents one embodiment of my invention;

Figure 2 is a curve which illustrates certain operating characteristics of the embodiment of Figure l; and

Figures 3 to 7, both inclusive, represent'other modifications of my invention.

In Figure 1, there is shown ay superheterodyne receiver for the reception of frequency modulated signals. This receiver includes a radio frequencyampliiier l0 having its input connected to an antenna.l system Ilk and I2 and its output connected to a frequency changer or oscillatormodulator i3. Waves from antenna ll are transferred successively through the frequency changer I3, an intermediate frequency amplifier li of one or more stages, a first amplitude limiter including electron discharge device. l5, a combined amplitude limiter and signal responsive control device including electron discharge device I 6, a frequency modulation detector including electron discharge device I1, a first audio frequency amplifier and silencing network including electron discharge device I8, an audio frequency amplier I9 of one or more stages, and

a sound reproducer 28.

It will be understood that the radio frequency amplifier IU, the frequency changer I3, the intermediate frequency amplifier I4, the frequency modulation detector including electron discharge device I'I', the audio frequency amplifier I8, and the sound reproducer 28 are of conventional construction and operation.

Considering briefly the operation of the receiver as a whole and neglecting for the moment the silencing network of this invention presently to be described, a desired received signal intercepted by the antenna II is selected and amplified by radio frequency amplifier I0, converted to a frequency modulated carrier signal in the frequency changer I3, amplified by the intermediate frequency amplifier I4, limited to a substantially constant amplitude by the cascaded limiters I5 and I6, and detected by the frequency discriminator detector including device I'I, thereby to produce an audio signal. The audio signal is then amplified in the audio frequency amplifier including electron dischargedevice I8, further amplified in the audio `frequency amplifier i i3 and reproduced by the sound reproducer 28.

The amplitude limiters including electron discharge devices I5 and I6, as shown, successively limit below predetermined intensities the amplitude of alternate half cycles of a wave from amplier I4. With this arrangement, self-'bias in the first limiter m-ay be eliminated, resulting in anode current cut-off in the first limiter for signal voltages only a volt or so on the negative side Iof the zero axis. The positive half cycles of the signal need not be limited in the first limiter, as they .are reversed in phase and applied Ito the second limiter grid as negative vol-tages. Since anode current cut-ofi in the second limiter occurs when the grid becomes negative by a small voltl age, effective clipping of the positive peak is .ob- .tained in Ithe second limiter. The structure of two limiters vconnected in .this manner is described and claimed in my copending application, Serial Number 371,606, filed December 26, 1940, and ma- .tured as Patent No. 2,323,880, dated July 6, 1943, for Wave amplitude limiting device, and assigned `to the assignee of .the present application.

The first limiter including elect-ron discharge device I5 is coupled tothe intermediate frequency amplifier by means of -a tuned radio frequency transformer system comprising primary inductyance 2|, secondary nductance 22 and capacitors 23 and 24. Suitable operating potentials for the pentode shown are provided by means Vof anode resistor 25, and screen voltage dropping resistor 26 -connected to the positive terminal of a source 21 of continuous voltage, the negative terminal of which is grounded. The screen of electron discharge device I5 is bypassed for radio frequencies by the capacitor 28, and the screen volta-ge is stabilized by .the resistor 29 `connected in parallel to capacitor 28. T-he anode .of electron discharge device !5 is coupled to the gri-d 38 of limiter device `i'5`by a capacitor 3l, which is shunted by a rev sistor 32. The signal output of the limiter device I6 is coupled to a conventional discriminator, or detector of frequency modulated waves, which in- `cludes the capacitor 33 and a 4tuned transformer comprising primary inductance 34, secondaryl nductance '35 and capacitors 36 and 31. The discriminator also include a pair of rectifier diodes I1, rectifier load resistors 38 and 33, and a high frequency bypass capacitor 48. Audio frequency voltage appearing across serially connected resistors 38 and 39 is applied to the grid 4I of the first audio amplifier I8 through capacitor 42, ad-

justable volume control resistor 43 and capacitor 44. The anode o f amplifier I 8 is supplied with operating vpotential through load resistor 45 connected to the positive terminal of source 21. The amplified audio signal developed on the anode of device I8 is transferred to amplifier I9 through capacitor 48.

Operating potentials for the combination limiter and signal responsive control device I6 are supplied from the positive terminal of source 21. Resistor 41 is connected between the screen and that positive terminal. Resistor 48, connected between screen and cathode, stabilizes Ithe screen voltage, and capaci-tor 49 bypasses the screen for high frequency currents. The anode of limiter I6 is connected .to the positive terminal of source 21 through the primary inductance 34 and resistor 58, a unidirectional voltage being developed between ground and a point 58' between resistor 5D and indue-tance 34. This unidirectional voltage is applied to a voltage divider comprising resistors '52 and 53 serially connected between ground and point 53'. Capacitor 54 is connected in shunt to resistor 53 across which a portion of the unidirectional voltage between ground and point 58' exists. I'his last voltage is impressed between the grid and cathode of amplifier I8 through a resistor 55. An adjustable high positive bias voltage is impressed on the cathode 56 of amplifier I8 by a voltage divider including resistor 51 connected between lthe cathode and the positive terminal of source 21, and by adjustable .cathode resistor 58, which is bypassed by capacitor '59.

In considering the operation of the circuit just described, i-t will first be assumed that no signal .of an appreciable amplitude is being received.

Under these conditions, the control grid 38 of limiter IB, which grid is connected to Ithe anode of limiter I5 by resistor 32, `is maintained at a slightly positive potential with respect to its grounded cathode. Resistor 32 is made large enough so that the voltage drop across it resulting from grid current is almost equal to the potential between the anode of limiter I5 and ground. The resulting small positive voltage on the grid 30 causes a relatively high anode current to flow 'through resistor 58. The consequent increased voltage drop across this resistor 50 reduces the potential at the anode of limiter I8 and at point 58 to a relatively low value. This reduced unidirecltional voltage appears across the voltage divider made up of resistors 52 and 53, and a predetermined part i-s applied to the grid 4I of audio amplifier I8 through a large grid resistor 55. The `cathode 56 of amplifier I8 is maintained at a positive potential, which exceeds that appearing on the grid under these no signal conditions by an amount sufiicient to cause cut-off of the anode current and thus render the amplifier inoperative.

The bias voltage on cathode 58 is readily adjustable to obtain this condition by adjustment .of adjustable resistance 58. Since in this embodiment of the invention the control voltage used for silencing is applied -to the grid of a sensitive audio frequency amplifier, that control voltage must be free of audio and radio frequency components. To that end the resistor 50, across which agr-mace the control voltage appears, is well' bypassed' b-y capacitor 5i; Resistor 52 and capacit-or'd provide additional ra-dio and audio frequency filter action. The time constant of the resistance and capacity combination in this control voltage di'- vider and audio frequency filter network is desirably such that the control voltage, and thus lthe silencing, or squelch, action, follow rapid manual tuning of th-e receiver in order to give inter-sta; tionl noise suppression as the receiver is tuned away from signals of satisfactory level.

In addition-to the action of the combined amplitude limiter and signal responsive control device Ilin limiting a received carrier wave, producing a rectified unidirectional voltage on the grid 3Q' of device IE, and amplifying `that unidirectional vcltage to produce a control voltage at the point 50", whereby in the absence of' acarrier wave the signal reproducer 28 is silenced or muted, the device I6 acts by reason of its reduced resistance between its anode and cathode in the absence of a carrier wave or in the'presence of a carrier wave of reduced intensity -to attenuate very `greatly voltages such. as carrier Wave or noise voltages ltransferred through the device it' to the frequency modulation detector including discharge device ll'. This attenuationmay be viewed vas the result of damping which causes mismatchingofthe anode resi-stance of device I6 with the inputimpedance of lthe frequency modulation detector, and which causes a great reduction in efficiency of the frequency modulation detector thereby, for a given. frequency shift of the carrier wave, less signal voltage is produced. Since ythis attenuation is produced whenever a carrier Wave of reduced intensity is received or Whenever noise voltage in the absence of a carrier wave is re-v ceived, such that a control voltage is produced at point 50', .two actions simultaneously assist each other in muting or silencing the receiver. One ofthese actions is .the muting or silencing of the receiver by .the control voltage developed at point 50', and the other is `the great attenuation of signals transferred from the device lSlto the frequency modulation detector and then to the signal reproducer 29. The cooperation of .these t'wosim-ultaneous actions results in a muting or silencing action of the receiver which "takes place over'aremarlrably small range of carrier wave in tensities appearing at the input of the receiver.

Assume now that a signal of satisfactory level is received. When a carrier Wave is appliedto the grid 30 of combination limiter and" signal responsive control device i6, the grid'assumes a negative bias. This negative grid bias increases with excitation because of the steady component of'rectified voltage across'resistor 32 arising from rectification of the Wave appliedl to gridV 3! through capacitor 3i. When thel grid becomes negative, the anode current of the limiter |6-`becomes relatively low and the voltage dropacross resistor 50 is greatly reduced. Since the potential at point v59 is equal to the voltage of source 21 minus the voltage drop across resistor Eil', the effective positive potential at thev point 5G" and therefore at the grid 4l ofv tube` l8- is increased. As has been heretofore explained, the cathode 56 of audio amplifier I8* operates Witlfa positive bias. However, when the .bias on the grid lll, which in' this case'is positive with respect to ground, approaches or becomes equal to the positive rbias on the cathode,` the amplifierbecomes operative.

It is desirable that, in the absence off a carfrier Wave on the grid 3i] of the limiterl ligithe vice gridy be' kept at a slightly positive potentiall with respect to` the cathode of limiter I S'. usual' conditions of reception, noise voltages are receivedv .by the receiver and appear on'A the grid' 30 of limiter If'` in sufficient intensity tobe rectified thereV and to maintain the grid 3U' ata slightly negative potential with respect to the cathode of limiter I6, unless means be providedto offset this' vslightly negative potential and maintain the grid positive in the presence of such noise voltages. Without suchmeans for maintaining the grid 3B positive, the impres-A sionV of a strong carrier wave' onthe gridv 30 would cause relatively little change in theanode current of limiter l6` and would reduce the usefulnessy of' the limiter as an amplifier for the control voltage developed on the grid 30 by rectification ofy an incoming carrier Wave.

Byproviding the means comprising resistors 32 andv 25 for maintaining grid 30 in theabsence of a carrier wave slightlypositive with respect` to thecathode of limiter I6, the space current of limiter |6-is maintained large in the absence of a-carrier, even in the presence of noise voltages of' substantial magnitude. Consequently,` the space current changes overa veryy large range from this large current to' a vsmall current when' a carrier Wave of satisfactory intensity is rectified upon'the gridf3flll and produces a control-voltage for amplification through limiter I6 to remove silencing or muting fromV the receiver. It must be borne in mind that, While the provision of means for maintaining grid 3U positive with respect to the cathode of limiter IG in the absence of a carrier Wave is desirable because it enhances the action of limiter i6 in amplifying the control voltage, limiter I6 nevertheless acts to some extent; in amplifying a rectified control voltage on the grid 30 even though such a positive voltageis not applied to the grid.

It is at present-believed that in large part of" maintenance of muting or silencing of the receiver in the presenceof large noise voltages,v

although such muting or silencing is removed i-nthe presence-of relatively small carrier Waves, is due to the action of the control device lf as a rather poor integrating circuit. It has been pointed out that rectication of noise and'vcarrier wave voltages ongrid 3d of thecontrol de- |6 takes place and that these rectified voltages are amplified through the device I6 to appear as control voltages at point 5D. In the presence of a carrier Wave continuously received, the rectied voltages on gridv 30, which voltages charge coupling condenser 3l so astochange the average voltage of grid 3ft, stay constant and are a function of the intensity of the received carrier Wave. On the other hand,` noise voltages of the type whose energy is' a discontinuous or irregular function of time are rectified onthe grid 3D and charge the condenser 3l but tov relatively a much lower average voltage because of the greater losses in the rectification circuit.

In other Words, instantaneous rectified voltages of high intensity appearing relatively infrequently on-the grid Si! are dissipated at a relatively greater rate than smaller rectified voltages. Therefore, assuming in one case that a, continuously received carrier Wave is impressed! on the grid 3l! and has a certain power, it produces a certain rectified voltage across condenser 3l. When a noise voltage, whose energy is received in` more or less discrete lumps,v is impressed? on the grid 30; the total power'being exactly?the'sameY as that forl the continuously2 Undef' received carrier wave, the rate of energy dissipation is much greater for the noise voltage, Whose peak intensities are necessarily greater than the intensity of the carrier wave` even though such peaks do not occur as frequently as half cycles of the carrier wave, and rectification ofthe noise voltage therefore produces a smaller average potential across the condenser 3I thanr does rectcation of the carrier waves.

It is desirable that the grid resistor 55 be mad high to prevent the impression of high positive voltages on the grid of amplifier I8 which would tend to introduce distortion on very strong signals. v

To summarize the operation of this embodiment of the invention, when a signal of satisfactory level is applied to the grid 3B of ythe limiter and signal responsive control` device IG, the grid ybecomes negative because of the component of rectified voltage across the resistor 32, the anode current drops `and a relatively high positive voltage appears between point 50 and ground. Part of this positive voltage is applied by means of an audio frequency lter and voltage divider network to thev grid 4I of first audio amplifier I8. This positive bias is approximately equal to a pre-established positive bias on the cathode 55 and the amplifier becomes operative.

When the signal disappears from the grid 3G of limiter-control device IS, the grid becomes positive because of its connection by resistor 32 to a source of high positive voltage, the anode current becomes high, the voltage drop across resistor 5B greatly reduces the potential applied to the voltage -divider 52 and 53 from point 59', and the grid 4| of amplifier I8 becomes less positive than the cathode 56 with respect to ground and ren-ders the audio amplier inoperative, thus silencing the receiver.

It may further clarify the operation of this invention to consider a speciiic embodiment 'for which circuit constants are given purely by way of illustration of one form of this specific embodiment. As illustrative of this specific form, the following circuit constants are given for the circuit of Figure 1:

With the constants shown the following operating conditions may be secured:

Strong No Signal Signa] D. C. voltage at the grid of limiter lo-.. 43% volts -6 volts D. C. voltage at point 50 +25 140 D. C. voltage at junction of resistors 52 and 53 +4. 0 22. 4

For the sake of clarity the explanation of thek silencing network of my invention has thusk far been limited to the extremeboundary conditions of no signal and strong signal. While these conditions actually exist with certain types of equipment, it is generally desirable that the silencing network hold the receiver inoperative until there appears a certain minimum level of signal which is necessary for satisfactory noise free reception. This minimum value of signal at which the receiver becomes operative is commonly referred to as the threshold level. The silencing network of my invention is arranged so that the threshold level may be adjusted over a wide range of input signals so as to meet satisfactorily various operating conditions. In the embodiment of Figure 1 this threshold level can be simply and effectively adjusted by varying the value of resistor 58, which action changes the cathode bias on audio amplifier I8.

While the silencing network of my invention could be operated with only the limiter and signal responsive control device I6 acting as a limiter and the preceding discharge device I5 operating as a carrier frequency amplier, it is preferred to use at least two limiters as shown in Figure 1. It is desirable to have other limiters in front of the limiter I6 because of the reduction in noise voltage applied to the grid of the limiter I6. When working with highly sensitive equipment of the type used in point to point communication, Weak signals are often accompanied by a substantial level of noise, which in some respects resembles the carrier signal in character. If this noise is amplified in the earlier stages of a receiver and applied to the grid 3l] of the limiter and control device I6 without prior limiting, it is rectified and tends to develop a voltage across resistor 32 and thus change the bias on the grid of the limiter.v This in turn changes the operating threshold point of the silencing circuits. It can be seen that the multiple limiter, in addition to its advantages when used in connection with conventional frequency modulation receivers, presents the added advantage when used with the invention of maintaining favorable operating conditions for the silencing circuits in the presence of a relatively high level of background noise.

In Figure 2, there is shown a graph of input signal intensities, plotted as abscissae, against corresponding anode voltages of limiter I B, plotted as ordinates. The values are those for a circuit whose constants are those of the illustrative example. Curvc I6' illustrates the measured relation between the two named parameters.

The receiver'of this example gives eiective amplitude limiting action when a signal of 7 microvolts or greater is applied to its input. When no signal is received, point 50 is at a positive potential of 25 volts with respect to ground. As the input signal is increased, the voltage with respect to ground at point 59' increases rapidly and rises to volts when an input signal of 10 microvolts is applied. It rises further to volts when the applied signal is 100 microvolts.

Owing to the steepness of the curve IE below 10 microvolts input, the threshold level for silencing may be satisfactorily set for a considerable range of signal inputs. That is, over this range of input signal intensity a minimum change of intensity in the input signal is sufficient to change the control voltage at point 5S from a condition in which the receiver is silenced to one in which the receiver is operative.

In Figure 3 is shown an embodiment of the invention which differs somewhat from the embodiment of Figure 1. In Figure 3 many elements are identical with those shown in Figure 1, and like elements inthe two figures have been given identical reference numbers. The limiters I and I5 are similar to fthose of Figure 1 except that resistor 68, which connects the grid 35 of the second limiter I6 to the positive `terminal of source2'l is used in place of resistor 32 which is in Figure v1 connected between the grid 30 of limiter I5 and the anode of limiter I5. In the rst audio stage which includes amplifier I8, an on-oif switch 6| for the muting circuit is .provided, which in the oh position connects the lower end of grid resistor 55 directly to the cathode 55 of amplifier I8. A diode 52 whose function is to prevent the grid Il of amplier I8 from becoming more positive than ythe cathode 56, is connected from the lower end of grid resistor 55 to a tap 58' on cathode resistor 58.

In operation, the circuit of Figure 3 is similar to Figure 1. The resistor 58 serves essentially the same function as the resistor 32 of Figure 1. That is, in the absence of signal, a positive voltage is applied to the grid 38 of limiter IB. This positive potential is small because the resistance of resistor 60 is high enough Aso that the voltage drop across resistorEIl, resulting from the flow of grid current through 50, is almost equal to the supply voltage of source 2l. When an appreciable signal is applied to the grid 3|) of limiter I5 through coupling capacitor 3|, the grid becomes negative because of the steady component of rectified voltage appearing across resistor 50.

It is sometimes desirable to be able to stop the operation of the noise suppression circuit. This is Iparticularly true in communications receivers. With lthe noise suppression circuit inoperative, it is possible to listen to `signals below the preestablishedthreshold level or to hear the set noise which exists in the absence of a satisfactory signal. The no signal roar of a sensitive receiver furnishes the operator with a rough indication that his receiver is performing satisfactorily.

For this purpose, the on-off switch 6| provides aconvenient means of eliminating-or restoring thenoise suppressor action. When the switch is open, the silencing network is in operation, and with the switch closed the lower end of grid resistor 55, to which is applied a portion of the control voltage from the point 50', is connected to the cathode 56 of amplifier I8, thus substantially eliminating the control voltage from the grid Vlll of amplifier I8 and causing the amplifier |8 to operate as a normal zero bias audio frequency amplifier regardless of the control voltage developed across capacitor 5| and applied to Voltage divider l52 and 53.

In the circuit of Figure 1 satisfactory operating conditions for theaudio frequency amplifier I8 are maintained, even though the control voltage appearing at the junction of resistors `52 and 53 and applied to grid resistor 55 becomes more positive than the positive Xed bias placed on the cathode 58 of amplifier I8. This result is achieved by using a grid'resistor 55 with a relativelyT high resistance. The voltage drop across the resistor when grid current flows in amplifier I8 is suicient to prevent the grid from ever becoming .objectionably positive with respect to the cathode. If itis desired to use a small grid resistor 55, or if it is necessary to prevent positive swing of the grid at'high signal levels, the diode 62,'connected as shown in-Figure 3, may be used. If the voltage appearing `at the junction of resistors 52 and 53 becomes positive with respect to'ltap 58 of resistor 58 connected to the cathode of amplifier i8, `the diode82, whose anode is connected to the junction of resistors 52 and 53 and whose vcathode is connected to the ltap 58', conducts and holds the potential at this point close to the'potential at tap-58', whichmay be adjusted to be at the potential of cathode 58 or may be negative with respect thereto.

It lis apparent, of course, that when the cathode of the diode v52 is at the voltage of a point on the cathode resistor 58 somewhere between the cathode vand ground, the grid cannot become more positive than a certain pre-established negative potential with respect to the cathode. For example, if the tap 58 be at a point on resistor 58 lsuch that the potential of the cathode of diode12 is one volt less positive than the positive bias on'the cathode 55 of amplifier I8, the potential on the grid 4| of amplifier :I8 is limited to a positive voltage which is approximately one Volt less positive'than the cathode potential of amplifier I8. That is, thegrid 8| of amplifier I8 must always be negative with respect to the cathode 56 by at least one volt.

The diode 52 and amplifier I8 may be in one envelope and may, if it is not desired to maintain grid 4| always ynegative with respect to cathode 58,-have a common cathode.

In Figure 4 is shown a circuit embodying another .form of the invention in which the lrst audio frequency amplifier is muted by reducing the Voltage applied to an electrode of the amplifier other than the control grid. Manyelements ofthe circuit are identical to those of Figure 1 and like elements in the gures have been given like reference characters. In this embodiment point 58', between which and ground the control voltage exists, is vconnected through a filter comprising resistor 63 and capacitors 5| and 64 connected between the ends of resistor 63 and ground, and .through an anode resistance 45 to the anode of amplifier I8, and through a resistor 15'` vto the screen grid of amplier I8. The grid return ypath for amplifier I8 indicates resistor 65, which is connected from grid 8| to cathode 5'8 of amplier I8. Capacitor 48 provides a path for the audio frequency signal from the volume control 53 to the rst audio frequency stage. Capacitor 86 provides a path for the audio frequency signal vfrom the output of amplifier I8 to succeeding laudio frequency stages or signal utilization apparatus.

WhileI have illustrated the audio amplifier I8 as a conventionally connected pentode in which the anode and screen grid voltages are supplied from point 58', it is to be understood that this control voltage may be supplied only to the screen grid or, if a triode amplifier be used, may be supplied only'to the anode thereof.

Theoperation of the circuit of Figure 4, whereby a control voltage is developed between point 58 and'ground, which voltage is a function of'input signal intensity, is the same as the operation described lfor Figures 1 and 3. In this embodiment iof my invention in Figure 4, the positive potential between point 50 and ground, which potential is high when a satisfactory signal is being received and low in the absence of signal, is smoothed'by capacitor 5| and further smoothed by the filter formed by resistor 83 and capacitor 54. The unidirectional voltage across capacitor 64 is used as the Voltage source for the audio amplifier I8. When no signal is being received,

. gain ofl the rst audio stage including amplifier lfashion.

This form of my invention, likevthe embodiment shown in Figure 1, is applicable to conventional frequency modulation broadcast receivers with only the addition of a few relatively low cost resistors and capacitors. Extra tubes are not needed and no' complex circuit changes or troublesome manufacturing adjustments are involved.

In Figure there is shown another embodiment of the invention which is similar in operation to that of Figure 1, and in which corresponding circuit elements have been given identical reference characters. In the circuit of Figure 5, the current or voltage change which takes place in the output circuit of the second limiter and signal responsive control device I6, operates a relay 19 whose contacts are connected to a signal translating channel of the receiver to silence the receiver. The use of an electromagnetic relay in the silencing circuit has the advantages that its operation is instantaneous when the threshold signal level is reached and that its characteristics can be controlled so that a time delay on the make or break of the contacts is secured. This latter feature is useful when it is required to tune -manually between two stations remote on the dial and hold the receiver silent for all intermediate stations.

The action of the circuit which results in a change of current in the anode circuit of limiter i6 and a change in the potential at point 5U has been explained in the descriptions of Figures 1 `and 3.

In the circuit of Figure 5 the relay 10 is connected so that the anode and screen current of the limiter I6, which current is supplied from source 21, passes through theelectromagnetic coil 1I of the relay. The relay contacts 12 are 4connected between ground and the ungrounded terminal of the sound reproducer 20, so that closing the relay contacts silences the receiver.

i Considering now the operation of the system just described, the relay 19 is adjusted so that, when a signal above the desired threshold level is present, the sum of the anode and screen currents passing through the electromagnetic coil 1I of the relay are insuiicient to close the contacts 12 and the receiver operates in a normal manner. Whenever the input signal drops below the threshold level, the anode current increases, as has been previously explained, and the relay acts to close contacts 12 which short circuit the sound reproducer 20 and thus render the receiver inoperative.

Figure 6 illustrates a Inuting relay 13 connected with an ampliiier 14 to produce silencing in response to changes of potential at point 50. While it is convenient and economical to place a relay directly in the supply voltage circuitof the limiter and control device I6, increased sensitivity is attained in the arrangement of Figure 6 by placing the relay in the output circuit of an amplifier which amplifies the control voltage at point 5D or other control characteristic developed by limiter and control device i6 and influences relay 13 correspondingly. Resistors 15 and 16 constitute a voltage divider across which the voltage between point 50 and ground is impressed. Capacitor 11 smooths the voltage across resistor 16 which voltage is applied between the grid and cathode of amplier 14. Voltage source 18 provides a positive bias for the cathode of amplifier 14 with respect to its grid and voltage source 21 provides the anode current for -amplier 14 which current operates the relay 13. Relay contacts 19 short circuit the sound reproducer 20 and thus render the receiver inoperative, and contacts 89 complete a circuit includingpilot lamp 8l and voltage source 82. Many elements in this Figure 6 are similar to those illustrated in previously described gures and are given like reference characters.

Considering now the operation of the circuit, of Figure 6 and referring particularly to such de` tails as distinguish it from circuits I, 3, 4 and 5, the positive bias on the cathode of amplier 14 is so adjusted that the amplier is operating at or near anode current cut-off when the voltage across resistor 1G is equal to that which exists whenv a signal at or near the desired threshold level is applied to the input of limiter IE. The anode current of amplifier 14 is then below the current required to open the contacts of relay 13, so that the normally closed contacts 19 short circuit the sound reproducer 20, silencing the receiver. Contacts 89 under the same conditions maintain the circuit 0f lamp 8| keeping it lit, and indicating that the receiver is in operation, even though no audio signal is heard.

When la signal above the threshold is applied to the input of limiter I'E, the positive potential across capacitor 5l increases and the positive potential on the grid of amplier 14 also increases, so that anode current ows, causing relay 13 to open contacts 19 and 80 to remove the receiver silencing and turn out pilot lamp 8l. While the lamp 3| is shown connected so that it lights when the receiver is silenced, it is clear that a simple change of relay contact connections makes it possible to have the lamp light when muting is removed in the presence of a satisfactory signal.

Figure 7 illustrates an embodiment of my invention in which silencing is achieved by rendering the frequency modulation detector inoperative. Many elements are similar to those of previous circuits and are given the same reference characters, The undirectional control voltage appearing across capacitor 5I is applied between ground and the center of the discriminator transformer secondary inductance 35 through a lter comprising resistor 90, capacitor 9i and resistor 92. The center lof inductance 35 is also connected to the junction of load resistors 38 and 39 through coupling capacitor 93. Resistors 94 and 95 serially connected from the positive terminal of source 21 to ground constitute a voltage divider. The Cathodes 96 of the double diode 91 are maintained at a positive potential with respect to ground by connecting them to an adjustable tap 98 on resistor 95. Other elements of the circuit of Figure 7 have been explained in the description `of previous circuits.

In operation, and referring particularly to the elements which are important in the operation of this embodiment of the invention, the positive bias voltage on cathodes 96, when no signal is applied to the input of limiter i6, is enough greater than the positive voltage between point 59' and ground, which voltage is applied to the diode anodes 99 through the filter comprising resistors and 92 and capacitor 9|, to make rectification in diodes 91 impossible and thus prevent i 13 the. development. .offl an audio vfrequency 1 signal across load resistors 38 and 3g.

When a signal of satisfactory levelis applied tothe input of limiter i6, thereris an increase in the positive voltage between point -50 and ground which results in an increase in gthe positive potential of diode anodes 99 so. -thatthey become at least as positive as cathodes 9B. This restores normal operation of the frequency modulation detector. Resistor Q2 serves to isolate the diode anodes 99, as well as the center of inductance 35, from ground for high frequencies. It is desirable that the other resistor 90 of the filter, taken together with resistor 92, be of high enough resistance to prevent lthe diode anodes yfrom assuming a potential so positive with respect to their cathodes 96 that appreciable discharge current is drawn. It is, howevennot desirable that resistor 92 be so large with` respect to resistors 38 and 39 that appreciable signal voltage is developed across resistor 92. This embodimentA of my invention is bestsuited for application in a frequency modulation receiver which is used for the reception of intermittent transmissions.

My invention has been explained in terms .of the-well known and extensively used frequency modulation system oi carrier signal transmission and reception. It is clear, however, from the description that the principles of my invention are equally applicable to any system using` any signal transmission system other than the commonlyused amplitude modulation system. The terms phase modulation, wave length modulation, and timing modulation are sometimes used to indicatecertain known. systems of carrier modulation `which in general depend on other than amplitude modulation, n

In fact, systems utilizing-modulation of various characteristics of pulses, including themodulation of the intensity thereof, may be used with myinvention, so long as thesystem utilizesmeans for limiting intensity of the carrierwave inthe receiver. When the term frequency modulated carrier waves or the like is used herein, the term signifies any carrier` wave which has some characteristic other than amplitudemodulated-in such a fashion that the wave may be limited in intensity and the signal modulated thereon may subsequently be reproduced. -It `is `to be understood that it is Within the scope of `my invention to use a suitable detector for any `particular type of such frequency modulated wave.

While I have shownand described particular embodiments of my invention, it will be obvious to those skilled in the art that changes and modifications may be made withoutdeparting from my invention in its broader aspects, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the :true spirit and scope of my invention.

I claim:

1. In combination, a source of a frequency modulated carrier Wave, a single electrondischarge device having a cathode, an anodepand a control electrode, and associated resistor means, arranged for rectifying said carrier Wave to produce a unidirectional voltage, for amplifying said undirectional voltage to produce a control voltage,v and for limiting the intensity of said carrier, wave below a predetermined intensity, means for utilizing a component of said carrier wave limited by said device, and means responsive to saidcontrol voltage for changing said utilizingmeans .from an inoperative condition to an operative condition.

Atra

.a2..';-I n.ccmb1naticn,assume-:cfa freenet-Camedulated carrier wave, a single electron discharge devieehavingla cathode, an anode, anda control electrode,` and, associated resistor means,` arranged tov be effective uponthe reception ofa carrier ,waveyoffgreater .than fa. predetermined intensity tov limit ,thezintensity of said kcarrier wave below a predetermined,intensity, and effective upon` the reception of any carrier Wave to rectify saidcarrierwave by utilizing said resistor means vto producea-unidirectional voltage-'and to amplify `said unidirectionalfvoltage to produce a-controlvoltage,means fordetecting and reproducing a signal yin,accordance With-,which a carrier wave limited 'by said device .is modulated, and means responsive to ,said controlvoltage; for changing :said detecting and reproducingmeans from an inoperative condition to an operative condition.

3..- -In combination, a-source of a frequencymodulated carrier .wave,-a single electron discharge device-.having atcathode, an anode, andvr a .control electrodaand associated resistor means, arranged for .rectifying said carrier wave to produce a unidirectional ,voltage for amplifying said'unidirectional voltageto produce a control voltage, and for limitingthe intensityof said carrierwaive below a predetermined intensity, meansfor utilizing saidy carrier. wavevlimited by ,said device. and means responsive' to said control voltagegfor altering operation lofsaid,utilizing means.

4. In combinatiomasourceof frequencynmodulated'carr-ierwavesa single Velectron discharge device having aca-thode,J an anode, anda control electrode,` and associated resistor means arranged for-limiting ,below alpredetermined intensity on said anodetheintensity of said carrier vwave impressed onsaid ycontrol. electrode, `means-including. said control electrode and.- said 1 resistor means for rectify-ing, saidcarrierwave toproduce al unidirectional voltage means for maintaining saidcontrolfelectrode at a,positive potential with respect to said cathode in theabsence of acarrier wave -Whereby noiseivoltageimpressed on. said control electrodeinthe absence of a carrier wave does-:noti substantiallyreduce lthe electron discharge betweensaid anode and cathode, means including said dischargegdevice for amplifying said unidirectional yvoltage to producel a control voltage onthe vanodeof said discharge device, means, Eo'r utilizing', said carrier wave on said anode, and means responsive to saidcontrol Voltage `for alteraring 'the performance of said utilizing means. Y

5l. vIn combination, a source of frequency modulated carrier waves, asingle :electron discharge device having a cathode, an anode, and a control electrode, and associated resistor means, arranged for limiting below; a predetermined intensity on said anode the,1`-ntensity of said carrier wave impressed on said .control ,'electrode, means including said controlelectrode and said resistor .means for rectifying saidcarrier Wave to produce a unidirectional voltage, mea-ns rfor-maintaining said control ,electrode at a positive potential. with respectto said lcathode, in the absence of a carrier wave wherebyv noise voltage impressed on said `control electrode in the absence of a carrier wave d oes not substantially reduce the electron discharge between saidanode and cathode, means including saidmdisch-arge device for amplifying said unidirectional voltage toproduce a control voltage on the .anode of said discharge device, means for utilizing said carrier-.wave on said anode, and v4`.means responsive to. ysaid control voltage for chneipssaidearrier Wave utilizing means from fi'S an inoperative condition to an opera-tive Acondition. i 6. In combination, a source of vhigh frequency 'voltages including noise voltages andcarrier wave frequency modulated voltages whose relative intensity varies from time to time, a single electron discharge device having a cathode, an anode,

and a control electrode, and associatedzresistor means, and arranged for limiting said frequency modulated voltages below a predetermined total intensity, for rectifying said "voltages by utilizing said resistor means to produce a unidirectional voltage, and for amplifying said unidirectional voltage to producea control voltage, means for reproducing a signal from a characteristic modulation of said voltages limited -by said device, and means responsive to anincreaseoi said control voltage above a predetermined intensity for changing said reproducing means from an inoperative condition to an operative condition,

7. In combination, a source of high frequency voltages including noise voltages and carrier wave frequency modulated voltages whose relative intensity varies-from time to time, a `single electron discharge device having a cathode, an anode, and a control electrode, and associated resistor means, arranged for-limiting -said voltages below a predetermined total intensity, for rectifying said voltages by utilizing said resistor means to produce a unidirectional voltage,fand for amplifying said unidirectional voltage to produce a control voltage, means for reproducing a signal from a characteristic 'modulation of said voltages limited by said device, means responsive to an increase of said control voltage above a predetermined intensity for changing said reproducing means from an inoperative condition to an operative condition, and means for adjusting the predetermined intensity'above which said control voltage causes'said last means to change said reproducing means from an Ainoperative to an operative condition, 1

8. In combination, a source -of affrequency modulated carrier wave, said source including a source ofy potential, a single electron discharge device having a cathode, an anode, and a control electrode and arranged for limiting below a predetermined intensity oln said anode the intensity of said carrier 'wave impressed on Said control electrode, means including said control electrode for rectifying said carrier wave to produce a unidirectional voltage, means'including said discharge device for amplifying said unidirectional voltage to produce a con-trolvoltage on the anode of said discharge device, means' inlcluding said potential source for maintaining said control electrode at a positive potential with re-A spect to said cathode in the absence of a carrier wave whereby a carrier wave rectined on said control electrode substantially reduces the electron discharge between said anode and cathode and ampliiies said unidirectional voltage in great degree, means for utilizing a component of said carrier `wave limited on said anode,'and means responsive to said control voltage for altering the performance of said utilizing means.

9. In combination, a source of` a frequency modulated carrier wave comprisingan electron discharge amplier device having an anode on which said carrier waves appear and on which a positive potential exists, a vsingle electron discharge device having a cathode, 4Van anode, and a control electrode and arranged for limiting below a predetermined intensity on saidL anode the intensity of said carrier waveimpressed on said control electrode, means including said control electrode for rectifying said carrier wave to produce a unidirectional voltage, means including said single discharge device for amplifying said unidirectional vol-tage to produce a control voltage on the anode of said single discharge device, means including a conductiveconnection between the anode of said amplier device and said control electrode for maintaining said control electrode at a, positive potential with respect to said catho-de in the absence of a carrier wave whereby a carrier wave rectified on said control electrode substantially reduces the electron discharge between said anode and cathode and amplifes said unidirectional voltage in great degree, mean-s for utilizing a component of said carrier Wave limited on said anode of said single discharge device, and means responsive to said control voltage for altering the performance of said utilizing means.

l0. In combination, a source of a frequency modulated carrier wave subject to interfering noise voltages, a single electron discharge device having a cathode, an anode, and a control electrode, and associated resistor means, arranged for rectifying said carrier wave to produce a unidirectional voltage, said discharge device being so arranged that a carrier wave in which the energy is substantially uniformly distributed produces a rectined voltage by said discharge device substantially greater than is produced by noise voltages of substantially the same energy content in which the energy is non-uniformly distributed with respect to time, said discharge device being arranged for amplifying said unidirectional voltage to produce a control voltage and for limiting the intensity of said carrier wave below a predetermined intensity, means for utilizing a component of said carrier wave limited by said discharge device, and means responsive to said control voltage for altering the performance of said utilizing means.

11. In combination, a source of a frequency modulated carrier wave, a pair of cascade connected electron discharge devices arranged for limiting the intensity of said carrier wave below a predetermined intensity, the second of said two cascade connected discharge devices being connected to associated resistor means and being arranged for rectifying said carrierv waves to produce a unidirectional voltage and for amplifying said unidirectional voltage to produce a control voltage, means for utilizing a component of said carrier wave limited by said second device, and means responsive to said control voltage for altering the performance of said utilizing means.

12. vIn combination, a source of a, frequency modulated carrier wave, resistor means, a single electron discharge device for limiting the intensity of said carrier wave below a predetermined intensity, said single electron discharge device having an anode and a cathode and being of the type in which large changes of voltage between said anode and cathode do not substantially aiect the discharge current therebetween, said discharge device being connected and associated with said resistor means and being arranged for rectifying said carrier wave to produce a unidirectional voltage and for amplifying said unidirectional voltage to produce a control voltage on said anode, means for utilizing a component of said carrier wave limited by said device, and means responsive to said control voltage for altering the performance of said utilizing means.

13. In combination, a source of a frequency carrier wavei-belovv" a' predeterminedintensity,

jsaid singlfeelectrn di-scharge device having a cathode, aco'ntrol electrode,l ascreen electrode,

an' anode', and associated 'resistorf means, said y ycarrierI wave being'impr'essed on said control elec- "anode,J said screen electrodebeingarranged to provide electrostatic shielding betweenfsaidv con- I trol electrode and saidanode, wherebylithe in- -ltensity' of'said limited carrier wave on 'said' anode I A v.is fnot undesi'ra-blvv increasedv by"'c"pacitive lcou- 'fpling of said unlimited 'carrier waveon'said conl 'trol electrode to s aidlanode,v said discharge device beingarranged for' rectifyin'g 'said-'carrier wave by'utili'zling said Vresistor mearisto produce 'af uni- `directional voltage'and for ampliiying' said'uni- Y fsaidutilizingmeans:

'V directional vvoltage to Aproduce a''control'voltage on 4said anode', said discharge devicefby reason of ther presence of the screen electrode between said ano-:le and said control electrode being ci the type in which large :changes Vcnfvoltage bey'twe'en said anode andI cathode v'do not"'substan tially aiect the discharge current' therebetween,

'whereby said amplifying means" amplies saidl 'unidirectionalvoltage greatly, means 'for utilizing f 'acomponent of- 'saidA carrier wave limited by said fdischarge'device; and mean-s responsive'to said control Ivoltage for altering the yperformance yof 14, In-combinatiomja' source of'fa'nlunlimited riving` a control voltage 'from s'aid carrier wave,

tivay of said utilization means wave below' a predeterrninedV intensity, means for ',"utilizing,saidlimited carriery wave,"jsaid`ilimiting meansfbeingjfarranged to reduce substantially 'the 'Sensitivityjof said utilizatior`1-'mear'isuponfl .reduction in intensity below said predetermined g' intensity of said unlimitedv carrier wave and lmeans responsive 'to said controlvolt'afgei i' makf ingsaid' utilization means inoperative substantially simultaneously with a reduction of sensil 153 In' combination`, a .source4 frequencyv modulated. carrier 'Wavjejfineansl for derivingfa control voltage from saidcarrierwave, Vmeans fozfflimiting.. the intensity of said Carrier Wave'below a 'predetermined intens'itvi,l means for detecting and 'reproducing asignal in accordjancewith which said .carrier vvaveis modulated,

said limitingmeans beingj arranged'toxir'educe substantially the sensitivityof` saidjdetecting and reproducing meansA upon reductionA iny intensity below said predeterminedfintensity of said unf limited carrier wave, and ymeansresponsive to said control voltage for"makingisaidfdetecting and reproducing' means inoperative substantially simultaneously withY a reductionjof sensitivity of 'said detection andA reproducing means.;` l6. In combination, a source of a carrier wave WH with a signal, means for limiting'theintensity ose .frequency is modulatedin accordance of said carrier wave below a predetermined int'ensity andfor deriving a control'voltageirom said carrier wave, means for detecting the signal "accordance with which the frequency' of said 'carrier wave is modulated and for reproducing` said signal',` means ,flor producing increasing amounts ,of attenuation oi said carrieruwave betiveenrsaid limiting means and said detecting means ,as the intensityo" said lcarrier "wave dejcreases below said predetermined intensity, and 'L means responsive to'saidcontrol'voltage:for mut- ",fff 1S l ingky said detecting and reproducing means substantially simultaneously with an. increase of attenuation of said carrierwave upon a reductionbelo'w saidpredetermined intensity.

17. In' combination, a source of frequency modulated carrier waves, tuning means* for selecting' a 'desired one' of said carrier waves of a particular frequency,-a single electron discharge device having va cathode', an anode, and a control 10 electrode, and associated resistor means, ar-

- ranged for'rectifying a selected carrier wave to producea'unidirectional voltage and for amplifying said unidirectional voltage toproduce a control voltage, said discharge device'being ar- 15 ranged for limiting below a predetermined inl tonsityisaidv selected carrier wave, means for reproducing a signal in accordance with which said selectedand limitedoarrier wave'is modulated, and means responsive to said controlvoltage to make said reproducing means operative to reproduce a signalfrom a selected and-limited carrior-wave only after a predetermined time after the appearance lof said control voltage,said predetermined time being sufliciently short that op- '25eration-of saidt'uning means' at normal speed makes said reproducing means inoperative at substantially `alltimes except when a carrier waveisselected. l

"2318. In"corribinational vsource of frequency modulated carrier waves, tuning means `for selecting a desired one of said-carrier waves of a particular frequency, a 'single electron discharge `device having a cathode, an anode`, and a control electrode, and associated resistor* means, ar- 35 ranged for rectifying avvselec'ted carrier wave to 'produce a unidirectional "voltag'e and for `ampliv`tying said unidirectionall voltage to produce a controlvolt-agasaid discharge device being ar- 'ranged for limiting below a predetermined invtensitysaidvselected'carrier wave, means for reproducing a signal `in accordance with which said selected and limited carrier wave is`m`odulated, and means responsive 'to said control voltage-to makev said reproducing means operative 451{tdreproduce'a'signal from a selected and'A limited 'carrier wave yonly after a predetermined' time 'after theappearance of said control voltage, said predetermined vtime being at least suicie'nt to prevent said reproducing means from becoming loperative in the presence of a transient voltage -from saidsource when no 'carrierwave is selected. 19."In combination, a source kof a frequency modulated carrier wave, a single electron discharge v device having ac'atho'de, an anode, and a control 'electrode fand Jassociated resistor means arranged for limiting theint'ens'it'y ofs'ad carrier "wave'belowja predetermined intensity, said dis- `charge device being arranged yfor rectifying said -"a1`-rierwa've by-futilizing'said'resistor means to 6 Tproduceaunidirectional'potential and for ampliiyin'grsaid unidirectional potentialftofiroduce a trolvoltage means `for 'detectinglandreproucing a 4signal in'bac'cordancewith which `said Limitedicarrewave'is mbdulaieasad detecting d "reprodllcng' means comprising'an electron lschar'sedgvce haring acathodef .control elec- ,.ifrofl'earl` landde; ,ndmarisfo' impssiiasad [detected signals between jsaid control' electrode and cathode to appear in' Inpliiiedintensity be- 70 rtween saidcathode and' jano'de," means'in',circuit between saidlcathodeand anode'l iorv adjusting the Detental cf, said athod `with,ressaliti),'said 1 sont!" :electrode whrebvjthe pcintmav' be adstd iiwhichsad discharge devietass t0 75' transfer saididetec't'edl signal' fromsaidcontrol electrode to said anode upon increasingly negative potentials on said control electrode with respect to said cathode, and means for impressing said control vvoltage on vsaid control electrode comprising a resistance in series between said amplifying means and said control electrode of suiiicient size to preventsaid control electrode from assuming a substantiallypositive potential i with respect to said cathode upon great increase of said control voltage, whereby greatchanges o f said amplified control voltage may be utilized to control said detecting and reproducing means withoutk `destroying said control electrode or causing substantial. distortion of said detected vrier wave below a predeterminedintensity, sai'd discharge device being arranged for rectiiying said carrier wave by utilizing said resistor means to produce a unidirectional potential and for amplifying said unidirectional potential to .pro-

duce a control voltage,means for detecting and reproducing a signal in accordance with which said limited carrier Wave is modulated, said detecting andV reproducing means comprising an electron discharge device having a cathode, a control electrode, an anode, and means for im- A duce a unidirectional voltage, for amplifyingsaid age, and .for limiting the intensity of said carrier .wave below 4a predetermined intensity, a second electron discharge device arranged for detecting .and reproducing a signal in accordance with which saidlimitedcarrier wave is modulated, said Vtained in a condition without electron discharge,

means responsive Lto said control voltage for impressing upon said second discharge device a polv tential with such polarity as to render said second. dischargedevice conductive whereby a signal f plied to said second discharge device produces uny., xdesirable lresults, and means comprising a unii laterallyconducting device so connected with said .second discharge device as to prevent the produc- .tion of such undesirable results by said bias potential.

V2 3. Inncombination, a source of a frequency *,modulated carrier wave, a single electron discharge devicehaving a cathode, an anode, and a .control electrode, and associated resistor means,

.arranged for. rectifying said carrier wave to produce a unidirectional voltage, for amplifying said pressing said detected signalsbetween said con- .and Y`reproducing a signal 1n accordance with llutvhich said limited carrier wave is modulated,

trol electrode and cathode to `appear in amplified intensity between said cathode and anode, means for manually adjusting the potential between y said control electrode and cathode whereby the point. may be `adjusted at which ysaid discharge device rceases to transfer; said detected signal from-said `control electrode to said anodeiupon electrode of suiiicient size to prevent said control lelectrode from assuming a substantially Apositive potential with respect to said cathode upon great increaser of said` control voltage, whereby great changes of saidamplied control voltage may be utilized to control saidA detecting and reproducing .l means without destroying saidl control electrode or. causing substantial distortion of said detected signal. e

21. In combination, a source of .a frequency modulated carrier wave, a single electron discharge device having a cathode, an anode, and a control electrode, and. associated resistor means,

` .arranged for rectifying saidcarrier wave to produce a unidirectional voltage, for amplifying said unidirectional voltage to produce a control voltagerandfor limiting the intensity of said carrier wave below a predetermined intensity, means for Hutilizingsaid carrier .wavelimited by said rst means, dischargedevice responsive to said control voltage for changing said utilizing means from an 4. inoperativecondition to yan operative.V condition,

' and means for manually disabling said last means n whereby said discharge device and said utilizing means may be operated without being affected by vsaid changing means.

22. In combination, asource of a frequency modulated carrier Wave, .a single electron discharge device having a cathode, an anode, and a control electrode. and associated resistor means,

.l f arrangedfor rectifying said` carrier wave to pro- .unidirectional voltage to produce a control volt- 30 age, and. for limiting the intensity of said carrier Wave below a predetermined intensity, a second electron discharge device arranged for detecting means, for.y maintaining said second discharge devicein nonfconductive condition to prevent repro- `duction oi asignal, and means for impressing said .control voltage .uponsaid second discharge de- -vice in .such polarity as to render said second v4.()4

discharge device conductive in the presence of a carrier wave and .the signal demodulated there- `.from,`whereby said detecting and reproducing means isl operative only in `the presence of a car- 24. In combination,l a source of a frequency modulated carrier wave, resistor means, a single electron discharge device being connected to said 1 resistor means f onrectifying said carrier wave to produce a unidirectionalpotential, for amplifying `said unidirectional potential to cause a decrease in the 4discharge current iiowing through said discharge device upon an increase in the intensity of said carrier wave, and for limiting the intensity of y, saidcarrier wave below a predetermined intensity, means for detecting and reproducing a signal inaccordance with which said limited carrier wave is modulated, and an electromagnetic relay ',responsive tothe discharge current through said discharge device responsive to an increase in the Qdischarge current through said discharge device for disabling said detecting and reproducing means Awhereby said reproducing means is opmeratiye, only inthe presence of a carrier wave.

. 2.5. IncOmbinatiOn, a source of a frequency modulated carrier wave, resistor means, a single electron `discharge device being connected to said resistor means for rectifying said carrier wave to produce a unidirectional potential, for decreasing Vthe discharge current flowing through said discharge device in response to an increase of said limitedcarrier wave, and an electromagnetic re- 2l lay responsive to a change in the discharge current flowing through said discharge device for producing a control operation.

26. In combination, a source of a frequency modulated carrier wave, means for deriving a control Voltage from said carrier wave, means for limiting the intensity of said carrier wave below a predetermined intensity, means for detecting a signal in accordance with which the frequency of said limited carrier Wave is mod ulated, means for reproducing said detected signal, means for producing increasing amounts of attenuation of said carrier wave between said limiting means and said detecting means as the intensity of said carrier wave falls below said predetermined intensity, and means responsive to said control voltage for making said detecting means inoperative substantially simultanea ously with an increase in attenuation of said carrier wave between said limiting means and said detecting means.

27. In combination, a source of a frequency modulated carrier wave, means for deriving a control voltage from said carrier wave, means for limiting the intensity of said carrier wave below a predetermined intensity, means com prising a second electron discharge path for detecting a signal in accordance with which the frequency of said limited carrier wave is modulated, means for reproducing said detected signal, means for producing increasing amounts of attenuation of said carrier Wave between said limiting means and said detecting means as the intensity of said carrier wave falls below said predetermined intensity, means for maintaining said second discharge path non-conductive in the absence of a carrier wave, and means responsive to said control voltage for making said second dis-charge path conductive substantially simultaneously with a decrease in attenuation upon approach of the intensity of a carrier wave to said predetermined intensity, whereby said reproducing means is operative only in the presence of a carrier and changes from operative to inoperative condition upon minimum change of carrier wave intensity.

28. In combination in a frequency modulation receiver including means for receiving a frequency modulated carrier wave and for limiting the intensity of said carrier wave below a predetermined intensity, and means for detecting and reproducing a signal in accordance with which the frequency of said carrier wave is modulated, said limiting means and said detecting and reproducing means each including at least one electron discharge path, means utilizing only the discharge path of saidv limiting means for rectifying a carrier wave to produce a unidirectional potential and for amplifying said unidirectional potential to produce a control voltage, and means utilizing only the discharge path of said detecting and reproducing means for disabling said detecting and reproducing means in response to a change in said control voltage resulting from a reduction in the intensity of said carrier wave.

29. In a frequency modulation receiver having a wave translating channel including in part a wave intensity limiting device, first means including a translating device to be operated to produce signal components of said wave in response to reception of waves by said receiver, second means normally rendering said first means inoperative for the reproduction of signal components, a source of frequency modulated waves arranged to be applied to said channel and to be limited in intensity bysaid wave limiting device, means for developing a control volttage in said limiting device in response to the appearance of the wave applied to said limiting device, said limiting device being arranged to amplify said control voltage, and means operated in response to the amplified control voltage for at least partially nullifying operation of said second means to thereby allow operation of said r-st means whereby said signal components may be reproduced.

CHALON WESLEY CARNAHAN.

REFERENCES CITED The following references are of record in the iile oi this patent:

UNITED STATES PATENTS Number Name Date 2,037,456 Burnside Apr. 14, 1936 2,049,750 Seeley et al Aug. 4, 1936 2,076,803 Van Slooten Apr. 13, 1937 2,078,055 Carlson et al Apr. 30, 1937 2,096,374 Beers Oct. 26, 1937 2,152,515 Wheeler Mar. 28, 1.939 2,179,974 Beers Nov. 14, 1939 2,182,677 Robinson Dec. 5, 1939 2,261,643 Brown Nov. 25, 1941 2,262,406 Rath Nov. 1l, 1941 2,263,633 Koch Nov. 25, 1941 2,296,101 Foster Sept. 15, 1942 2,301,620 Fowler Nov. 10, 1942 2,316,902 Trevor Apr. 20, 1943 2,370,216 Worcester, Jr Feb. 27, 1945 2,372,934 Campbell Apr. 3, 1945 Certificate of Correction Patent No. 2,447,564. August 24, 1948.

CHALON WESLEY CARNAHAN It is hereby certied that errors appear in the printed specification of the above numbered patent requiring correction es follows: Column 4, line 2, for include read rlnclades; column 5, line 33, for thereby read whereby; column 10, line 43, for f indicates read includes; column 12, line 49, and column 13, lines 67 and 68, for undirectional read anidirectzonal; column 19, line 63, after rst insert discharge dem'ce,; line 64, strike out discharge device; column 22, line 15, for produce read reproduce; lines 22 and 23, for volttage read. voltage; and that the said Letters [SEAL] THOMAS F. MURPHY,

Assistant Uommz'm'oner of Patents. 

